Golfers prefer golf clubs that exhibit performance characteristics such as forgiveness and playability. One measure of “forgiveness” can be defined as the ability of a golf club head to reduce the effects of mis-hits, e.g., hits resulting from striking the golf ball at a less than ideal impact location on the golf club head, on flight trajectory and shot distance. Greater forgiveness of the golf club head generally equates to a higher probability of hitting a straight golf shot. “Playability” can be defined as the ease with which a golfer can use the golf club head for producing accurate golf shots.
Golf club forgiveness is directly affected by the moments of inertia of the golf club head. A moment of inertia is a measure of a club head's resistance to twisting about the golf club head's center-of-gravity, for example on impact with a golf ball. In general, a moment of inertia of a mass about a given axis is proportional to the square of the distance of the mass away from the axis. In other words, increasing the distance of a mass from a given axis results in an increased moment of inertia of the mass about that axis. Higher golf club head moments of inertia result in lower golf club head rotation on impact with a golf ball, particularly on off-center impacts with a golf ball, e.g., mis-hits. Lower rotation in response to a mis-hit results in a player's perception that the club head is forgiving. Moreover, higher moments of inertia typically result in greater ball speed on impact with the golf club head, which can translate to increased golf shot distance.
United States Golf Association (USGA) regulations and constraints on golf club head shapes, sizes and other characteristics tend to limit the moments of inertia achievable by a golf club head. For example, the highest moment of inertia (Izz) allowable by the USGA is currently 5,900 gcm2 (590 kgmm2).
Because of increased demand by golfers to hit straighter and longer golf shots, golf club manufacturers have produced golf club heads that increasingly approach the maximum allowed moment of inertia (Izz). Although golf club heads with high moments of inertia (Izz) may provide greater left-to-right shot shape forgiveness, such benefits are contingent upon the golfer being able to adequately square up the club face prior to impacting the golf ball. For example, if the golf club head face is too open on impact with a golf ball, the ball will have a tendency to slice. The harder it is to rotate the golf club head during a swing, the more difficult it is to square the golf club head prior to impact with a golf ball and the greater the tendency to hit errant golf shots. Often, the bulkiness or size of a golf club head can negatively affect the ability of a golfer to rotate the golf club head into proper impact position. In other words, because the mass of bulkier golf club heads is distributed further away from the hosel and shaft, the moment of inertia about the shaft is increased making it harder to rotate the golf club head about the shaft during a swing.
Conventional golf club heads approaching the maximum allowable moment of inertia (Izz) tend to be bulkier than club heads with lower moments of inertia due to the outward extend of the periphery of the golf club head. Although the bulkiness of the golf club heads may provide a higher moment of inertia (Izz) for greater forgiveness, the golf club head makes it harder for a golfer to square up the golf club head. In other words, the high forgiveness of the golf club head can decrease the playability or other characteristics of the golf club head.